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The reduction of head-mounted display optics to a scale commensurate with incorporation within eyeglasses has been addressed by several inventors. One method involves free-space image projection from the temple toward the eyeglass lens, the rays then being reflected from the inside surface of the eyeglass lens toward the eye. For example, Upton (U.S. Pat. No. 3,936,605) describes an eyeglass display in which the rays are reflected from miniature mirrors placed on the inside surface of the eyeglass lens (FIG. 1). Bettinger (U.S. Pat. No. 4,806,011) discloses an improvement on the Upton invention in which the inside or outside surface of the eyeglass lens itself, having a curvature intended to correct the user""s vision, is used for focusing the light from the image and for reflecting the light to the eye. Spitzer discloses an eyeglass display system in which the light is supplied by an optical fiber to a micromechanical image source appended to or placed within a temple or other fixture, and which is then reflected from the surface of the eyeglass lens (U.S. Pat. No. 5,715,337).
A number of patents have also described how embedded optical surfaces can be used in display viewing systems. Reichert (U.S. Pat. No. 5,654,827) discloses an optical system for a head-mounted display in which light is propagated to a surface within the lens, reflected and magnified and then directed to the eye of the user (FIG. 2). In another invention by Spitzer, light from the display is relayed through the body of the eyeglass lens and then passes from an interior partially reflecting surface within the lens to the eye (U.S. Pat. No. 5,886,822, the disclosure of which is incorporated by reference herein).
In prior art inventions of Bettinger, Reichert and Spitzer, the inventors make use of on-axis or nearly on-axis optical systems. An on-axis optical system means an optical design in which light is incident on optical surfaces having rotational symmetry about the optical axis. A nearly on-axis optical system means a system that is close enough to axial so as to not require correction of aberrations induced by the off-axis geometry. In an off-axis design, the resulting aberrations are visible to the average viewer unless some form of aberration correction is also employed.
The concepts of on-axis (or axial) and off-axis (or non-axial) systems are simple to define for reflective curved surfaces, because these surfaces have a center of curvature and a vertex and in general have rotational symmetry about a line connecting the center of curvature and vertex. A system is axial if a ray from the center of the object (or display) passes through the center of curvature of the reflective surface and also passes through the center of the pupil of the eye. The distinction between axial and non-axial systems is more fully described by G. H. Smith in xe2x80x9cPractical Computer-Aided Lens Designxe2x80x9d (Willmann-Bell).
The importance of off-axis systems relates to the fitting of an optical system within eyewear close to the head. Off-axis designs have the ability to form an optical path that more closely conforms to the shape of the head, and hence have the potential to produce a more compact system than axial systems.
A principal limitation of approaches that use the external surfaces of the lens for image magnification and reflection, such as the approaches of Bettinger and Upton, arises from the primary need in an ophthalmic device such as eyeglasses to use the external surfaces of the lens for vision correction. In general, the external surfaces cannot be shaped to provide simultaneous vision correction, magnification for display viewing and aberration correction.
Prior art inventions have considered off-axis optical systems. For example, the invention of Upton involves use of off-axis reflectors, and Bettinger (U.S. Pat. No. 4,806,011) uses an optical design that may be either nearly on-axis, or off-axis, depending on the position of a relay mirror. Spitzer (U.S. Pat. No. 5,715,337 and U.S. Pat. No. 5,886,822) may be axial or non-axial depending on the internal configuration of the surfaces. Except for Spitzer, these prior art off-axis systems employ reflection from exterior surfaces of the eyeglass lens. In the Upton invention, the eyeglass lens is provided with an exterior array of tiny mirrors and in the Bettinger invention the lens surface is coated with a partially reflective mirror coating.
Rallison (U.S. Pat. No. 5,303,085) has described a method of using a holographic combiner mounted between two thin curved surfaces to form an off-axis combiner for a head-mounted display. This technique does not use reflection from the lens surface, nor does it use refraction from the lens surface. Owing to the off-axis nature, a fiber optic faceplate is necessary to correct aberrations.
The invention described herein comprises an optical system for viewing a display which is an improvement over the prior art, owing to the integration of the off-axis free-space projection of an image to the surface of a lens, refraction by the surface of the lens, propagation of the rays to the interior of the eyeglass lens, and then reflection and magnification of the image from an interior eyeglass lens surface, followed by reflection to the surface, a further refraction at the surface, and propagation of the light to the eye. Generally, on-axis or nearly on-axis optical approaches yield the lowest aberration. However, aberrations resulting from off-axis designs can in many cases be corrected in various ways to be described, and with such correction, an off-axis design can lead to a display viewing system of reduced weight and complexity. However, the use of interior optical surfaces need not affect the user""s vision and, therefore, can be used for magnification and aberration control.
The present invention is an improvement of the prior art because optical elements that are provided inside the eyeglass lens are coupled to exterior optics by a significant degree of refraction at the surface of the eyeglass lens. The improvement over Reichert""s head-mounted display viewing optics (U.S. Pat. No. 5,654,827) and Spitzer""s eyeglass display optics (U.S. Pat. No. 5,886,822) is the use of an off-axis optical design, coupled with refraction from the surface of a lens having high refractive index, and combined with surfaces internal to the lenses. This invention leads to a substantial reduction in the degree of deviation from axial optics, and a commensurate reduction in aberrations.